Another and simpler form is shown in [Fig. 42], where the ball H is mounted on the end of a lever L pivoted at P, its rise or fall completing the circuit of B and C as before.
Where it is desired to give a different signal for the rise and the fall of level, two bells B and E ([Fig. 43]) may be used connected as shown. The rising of the ball will ring bell B, and its fall, bell E.
Fig. 42
Fig. 43
In both forms of indicator, a means must be provided that an undue rise may not bend the lever. This may be accomplished by using contact springs instead of contact screws; it is, however, then harder to adjust the indicator to fine differences of level.
In all cases the contacts must be faced with platinum to prevent corrosion.
Burglar Alarms.
A burglar alarm is a device for indicating the opening of a door or window, by the ringing of a bell or operation of an annunciator. The contact apparatus at the points to be protected may either open an electrical circuit or close one, in the latter case being mere modifications of push buttons. The simplest form is the latter or open-circuit method.